Hawkes P.W., Spence J.C.H. (Eds.) Science of Microscopy. V.1 and 2

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1100 U. Weierstall

line scans across the oscillating LDOS at very low bias voltage perpen-

dicular to step edges on Ag(111) have been measured in the temperature

range of 3.5–178 K and compared with theoretical calculations. By mea-

suring at the Fermi energy, the lifetimes are not reduced by e–e scatter-

ing (since the e–e lifetime is large at E

). The measurements showed that

the temperature-dependent damping of the standing waves at step

edges could be very well described by Fermi–Dirac broadening alone.

Therefore only a lower limit for the e–ph lifetime could be given, point-

ing to signiﬁ cantly longer lifetimes than all previous data.

Below 1 eV the phase relaxation length L

becomes longer and longer

and the damping effect for LDOS oscillations at step edges becomes

small. Measuring the additional damping of the oscillating LDOS

caused by L

beyond the inherent

1 kx

decay at energies close to the

Fermi level is very demanding. It has been proposed that by choosing

a closed scattering geometry such as a quantum corral assembled with

the low-temperature STM, reﬂ ections add up and contribute to a high

total intensity, which allows the measurement of small effects. This

idea was realized and the lifetime of surface-state electrons conﬁ ned

in an artiﬁ cial atomic structure has been measured at various energies

on an Ag(111) surface at 6 K (Braun and Rieder, 2002). Ag atoms were

taken out of the surface by applying short voltage pulses to the tip. By

means of lateral manipulation, single Ag atoms were then assembled

along closed-packed row directions to form a triangle with a base

length of 24 nm as shown in Figure 17–26. The surface-state electrons

are scattered by the positioned Ag atoms, resulting in a standing wave

pattern as can be seen in Figure 17–26. For the lifetime determination,

dI/dV maps were recorded in constant height mode inside the triangle.

Lifetimes have been determined by ﬁ tting theoretically calculated dI/

dV maps to the experimental maps that have been recorded in the

energy range of −55 to +796 meV with respect to the Fermi level. The

model to calculate the dI/dV maps contained three ﬁ t parameters:

the phase shift and absorption of the electron wave during scattering

events and the phase-relaxation length. The extracted τ(E) data also

reproduced the τ ∝ E

−2

law predicted by Fermi liquid theory for a 2DEG

(Pines, 1966). Electrons injected at an energy E scatter inelastically with

other electrons into unoccupied states with an energy smaller than E.

This results in a singularity of the lifetime at the Fermi energy due to

the reduction of the number of available states into which to scatter. In

accordance with theory, all STM lifetime measurements showed a

sharp maximum at the Fermi energy.

3.1.5 Stark Effect

The inﬂ uence of the high electric ﬁ eld between the STM tip and the

sample on STS spectra has been measured in an LT-STM study (Kroger

et al., 2004). In metals, the electric ﬁ eld is efﬁ ciently screened by the

conduction electrons. Nevertheless, the wavefunction of surface state

electrons is evanescent into vacuum and can thus be affected by the

electric ﬁ eld. By varying the electric ﬁ eld of the microscope through

the resistance of the junction from 1 GΩ (low ﬁ eld) down to 60 kΩ (high

ﬁ eld), it could be demonstrated that the STS spectra of the Au(111) and

Chapter 17 Low-Temperature Scanning Tunneling Microscopy 1101

Cu(111) surface states undergo a downward shift that was attributed

to the Stark effect. The presence of a Stark effect in the noble metal

surface states even at usual tunneling parameters suggests that this

effect is quite common for STS.

3.1.6 Long-Range Interactions between Adatoms

The previous studies on standing wave patterns in the electron density

of surface electron states concentrated on the effects caused by the

adatom scatterers on the surface-state electrons. On the other hand,

surface-state electrons give rise to an interaction between the scatter-

Figure 17–26. Series of images showing the construction of the triangle con-

sisting of 51 Ag atoms on an Ag(111) surface (scan area 49.3 nm × 49.3 nm).

(From Braun, 2001.)

1102 U. Weierstall

ers. This surface-state-mediated interaction is long ranged and oscilla-

tory in nature. Quantitative measurements of the long-range interaction

energy between single Cu adatoms on the Cu(111) surface have been

performed with a low-temperature STM operated at 9–21 K (Repp

et al., 2000). The main criteria of the theory of surface-state-mediated

interaction energy (Lau and Kohn, 1978) could be reproduced in the

experimental data: the interaction energy is oscillatory with a period

of λ

/2, where λ

is the Fermi wavelength, and the envelope of the

magnitude decays as 1/d

, where d is the adatom separation.

3.1.7 Magnetic Atoms at Surfaces: Kondo Effect

The smallest magnetic structure in condensed matter physics is a single

magnetic atom in a nonmagnetic host. The localized spin of the mag-

netic atom interacts with the spin of the surrounding delocalized con-

duction electrons. For temperatures below a characteristic Kondo

temperature T

, this interaction causes the conduction electrons of the

host metal to condense into a many-body ground state that collectively

screens the local spin of the impurity. This screening cloud exhibits a

set of low-energy excitations called the Kondo resonance. Due to the

formation of the screening cloud, the LDOS near the Fermi energy is

enhanced at the site of the impurity. The Kondo resonance disappears

at temperatures above T

, and it energetically splits by the Zeeman

energy in an applied magnetic ﬁ eld (Hewson, 1993). The Kondo

resonance shows up as a peak at the Fermi level in high-resolution

photoemission spectroscopy.

This resonance can be probed locally by STS and manifests itself as

a sharp (∼10 meV wide) depression in the differential conductivity at

the Fermi energy. The feature is localized to within ∼1 nm of the Kondo

impurity, and is observed only at low temperatures. Co impurities on

Au(111) have been studied (Madhavan et al., 1998) at 4 K. The Kondo

resonance line shape has been interpreted in terms of quantum inter-

ference resulting of two possible tunneling channels, one direct channel

into the Co d-orbital and another channel into the surrounding con-

tinuum of conduction band states. A similar observation has been

made for Ce adatoms on Ag(111) (Li et al., 1998c). A later experiment

with Co adatoms on Cu(100) and Cu(111) (Knorr et al., 2002) showed

that while at the Cu(111) surface both tunneling into the localized

impurity state and into the substrate conduction band contribute to the

Kondo resonance, tunneling into the conduction band dominates for

Cu(111) (Figure 17–27).

Another landmark STM experiment showed that when a magnetic

atom is placed at one focus of a properly sized empty elliptical corral

built from magnetic atoms, a “mirage” of the Kondo signature in the

LDOS is cast to the opposite empty focus (Manoharan et al., 2000) more

than 7 nm away. The experiments were performed with Co atoms on

Cu(111) with a low-temperature STM at 4 K well below the Kondo tem-

perature of the system. Co atoms were evaporated onto the clean Cu

surface at 4 K. The ellipses where constructed by use of the adatom

sliding process (Eigler and Schweizer, 1990; Stroscio and Eigler, 1991).

Figure 17–28 shows the results of these measurements, simultaneously

Chapter 17 Low-Temperature Scanning Tunneling Microscopy 1103

acquired constant current images, and dI/dV maps of the corrals. dI/dV

maps measured on the corrals without an interiour atom have been

subtracted from the maps measured with an interior atom to remove

the contribution of the LDOS oscillation and emphasize the Kondo

signal. The constant current images (Figure 17–28a and b) show the

location of the Co atoms and oscillations in the LDOS due to two-

dimensional conﬁ nement of the surface-state electrons whereas the

differential conductance spectra show the Kondo signature of Co inside

the corral. The striking results reveal two interior positions that show a

Kondo resonance: one signal is centered on the real Co atom at the left

focus, while the other signal is centered on the empty right focus. The

localized electronic structure has been projected from the occupied

focus to the unoccupied focus. Tunneling spectra taken at both foci

showed that the mirage at the right focus is a faithful spectroscopic

replica of the real atom at the left focus attenuated by a factor of eight.

An energy-dependent phase shift of electrons scattering off mag-

netic impurities has been shown to explain the observed quantum

r =

1.4

1.3

1.2

1.1

1.0

–20 –10 0 10 20

Sample bias (mV)

[dI/dV] / [dI/dV]

V = 0

Tip

Cu(111)

Figure 17–27. Spectroscopic signature of the Kondo resonance around a single

Co atom on Cu(111) Tunneling spectra shown have been acquired over the Co

atom with increasing lateral displacement r. The spectroscopic feature is

narrow (9 mV FWHM) and dies off over a lateral length scale of 1 nm. Measure-

ments with different tips showed nearly identical features. (From Manoharan

et al., 2000. Reproduced by permission of IBM Research, Almaden Research

Center.)

1104 U. Weierstall

mirage (Fiete et al., 2001) and Schneider et al. (2002) have shown experi-

mentally that this phase shift can also be accurately determined with

low-temperature STM when the Co atoms are simply adsorbed on

Ag(111) and are not placed in artiﬁ cial resonators.

3.1.8 Fermi Contour Imaging

It has been demonstrated that with a low-temperature STM an image of

the surface Fermi contour is directly obtainable by Fourier transform-

ing a low-bias constant-current image of the standing waves in the

LDOS (Petersen et al., 1998, 2000ab; Sprunger et al., 1997). The wavevec-

tors of electrons with an energy at the Fermi level are conﬁ ned to the

surface Fermi contour, and consequently standing wave patterns

observed in low-bias STM images contain information about the entire

Fermi contour. This information can be extracted by performing a

simple two-dimensional Fourier transform of the STM image. Thus the

power spectrum of a low-bias STM image contains an image of the

surface Fermi contour. For an isotropic free electron like surface state a

ring is observed in the Fourier transform of the oscillating LDOS image,

Figure 17–28. Quantum mirage. STM topographs of two elliptical corrals with

(a) a Co atom at one focus and (b) a Co atom off focus. Associated dI/dV maps

showing in (c) the Kondo effect projected to the empty right focus, resulting

in a Co atom mirage; in (d) the mirage vanishes. (From Manoharan et al., 2000.

Reproduced by permission of IBM Research, Almaden Research Center.)

Chapter 17 Low-Temperature Scanning Tunneling Microscopy 1105

as shown in Figure 17–29. The radius of this ring is twice the Fermi wave

vector 2k

. This is due to the fact that waves in the charge density are

observed rather than in the actual wavefunction. Application of this

method to anisotropic surfaces was undertaken by Hofmann et al.

(1997), who studied the Be(101

0) surface. An image of the standing

waves on Be(101

0) recorded at 4 K is shown in Figure 17–30a. In contrast

to isotropic surfaces, standing wave patterns on Be(101

0) are visible

only when originating from steps in the Γ

M direction, whereas no

waves from the steps in the Γ

direction are present. The two-

dimensional Fourier transform of the standing wave pattern shows the

Fermi contour at the SBZ boundary as half ellipses. Obviously the

screening of defects at the surface is highly anisotropic due to the ori-

entation of the Fermi contour. This is a beautiful example of the intimate

coupling between the Fermi contour and the screening at the surface.

3.2 Single Atom and Molecule Manipulation

The tip of an STM always exerts a force on an adsorbate atom or mol-

ecule. This force contains both van der Waals and electrostatic contri-

butions. By adjusting the position and the voltage of the tip, the

magnitude and direction of this force may be tuned, thereby enabling

atomic scale manipulation processes. Two basic modes of transport of

atoms or molecules have been identiﬁ ed: lateral manipulation, i.e.,

moving particles along the surface by which they maintain contact

with the substrate, and vertical manipulation, by which the particles

are picked up by the tip and released back to the surface at a desired

place. To realize a lateral manipulation experiment, the tip is posi-

tioned on the adsorbate and its height reduced to 0.2–0.4 nm by reduc-

ing the tunneling resistance from ∼1 MΩ to 100 kΩ. The manipulation

is then performed in constant-current mode and the particle is dragged

Figure 17–29. Imaging the Fermi contour. (a) Constant-current image of the

Be(0001) surface at 150 K showing LDOS oscillations. (b) Two-dimensional

Fourier transform of (a) showing a ring-shaped Fermi contour as expected for

an isotropic free electron-like surface state. The spots are reciprocal lattice

vectors and scale reciprocal space. (From Petersen et al., 2000a.)

1106 U. Weierstall

by lateral movement of the tip to the ﬁ nal position. As single metal

atoms or small molecules are mobile at ambient temperatures, experi-

ments on small adsorbates located on low index metal surfaces have

been conducted at low temperatures. Using an LT-STM reduces or

eliminates thermally activated diffusion of adatoms on the surface and

enables the experimenter to build stable nanostructures or synthesize

molecules atom by atom. The electronic properties of these artiﬁ cial

structures can then be studied by STS.

Reliable lateral manipulation and build-up of extended nanostruc-

tures on an atomic basis with adsorbed atoms and small molecules at

low temperatures have been demonstrated by several groups. Don

Eigler’s group at IBM-Almaden assembled arrays of Xe atoms on Ni(110)

at 4 K (Eigler and Schweizer, 1990) (Figure 17–31). An atomic switch has

Figure 17–30. Imaging the Fermi contour. (a) Constant current image of the

Be(101

0)surface at 4 K with a surface step. Anisotropic scattering at step edges

results in LDOS oscillations mainly in one direction. (b) Logarithm of the

power spectrum of the image. The elliptically shaped features originate from

the Fermi contour of the anisotropic surface states. Inset shows a real space

top view of the surface and the surface Brillouin zone with the Fermi contour.

(From Hofmann et al., 1997.)

Chapter 17 Low-Temperature Scanning Tunneling Microscopy 1107

been realized where a xenon atom is moved reversibly between two

stable positions on the STM tip and a nickel surface (Eigler et al., 1991).

Heat-assisted electromigration (Ralls et al., 1989) has been identiﬁ ed as

a possible explanation for the vertical manipulation of a xenon atom

between a nickel surface and the tip (Eigler et al., 1991). Several other

possible mechanisms responsible for lateral and vertical manipulation

processes have been investigated (Stroscio and Eigler, 1991). Field-

assisted diffusion, which relies on the static and induced dipole moment

of the adatom in the strong electric ﬁ eld under the tip, has been identi-

ﬁ ed as being responsible for lateral manipulation of Cs atoms on

GaAs(110) at room temperature (Whitman et al., 1991). The lateral

manipulation of Xe on Ni(110) has been attributed to the chemical

bonding force between the adatom and the tip, since it is not sensitive

to the sign or the magnitude of the electric ﬁ eld, the voltage, or the

current. It depends only on the separation of the tip from the substrate

(Stroscio and Eigler, 1991). A possible mechanism for vertical manipu-

lation of adatoms is transfer on contact, whereby the tip is moved close

to the adsorbate until the energy barrier separating them is lowered

enough that thermal activation is sufﬁ cient for atom transfer. This

process does not need an electric ﬁ eld, whereas ﬁ eld evaporation,

another possible mechanism, relies on the strong ﬁ eld to create ions

that are evaporated over the Schottky barrier. Inelastic tunneling,

which leads to multiple vibrational excitations of the atom–surface

bond until the bond is broken, has been used to explain the desorption

of hydrogen atoms along single dimer rows of the Si(100)-2 × 1 surface.

Linewidths as small as 1 nm have been achieved (Foley et al., 1998) in

these experiments. Inelastic tunneling was also used to selectively cut

single molecular bonds in O

on Pt(111) (Stipe et al., 1997b).

Lateral manipulation of Cu atoms and Co and C

molecules on

Cu(211) in constant current mode has been demonstrated by Meyer et

al. (2001) at 15 K. The measured tip height curves during pulling of a

Cu atom along a close-packed row on Cu(111) showed characteristic

Figure 17–31. Patterned array of xenon atoms on an Ni(110) surface. The

atomic structure of the nickel surface is not resolved. Each letter is 5 nm from

top to bottom. (From Eigler and Schweizer, 1990. Reproduced by permission

of IBM Research, Almaden Research Center.)

1108 U. Weierstall

jumps due to the atom hopping from one fcc adsite to the next. By

recording the tip height during adatom manipulations, it is possible to

distinguish three different types of lateral manipulation: pushing,

pulling, and sliding (Bartels et al., 1997a). During a pulling process the

adsorbate is situated behind the tip apex with respect to the manipula-

tion direction (attractive tip–adatom interaction). By applying larger

forces than for pulling, the adsorbate remains under the tip apex

without escaping sideways from the tip trajectory (sliding mode, attrac-

tive tip–adatom interaction). Small molecules can be manipulated by

the pushing mode, where the adsorbate is in front of the tip (repulsive

tip–molecule interaction).

Bias polarity-independent STM-induced desorption of individual

molecules from Cu(111) has been demonstrated at 15 K, which

sometimes leads to transfer of the molecule to the tip (Bartels et al.,

1999). Vertical manipulation of C

molecules at low temperatures has

also been shown (Meyer et al., 1996). Pb monomers and dimmers on

Cu(211) have been moved laterally at 20 K. Vertical manipulation of CO

molecules on Cu(111) has been shown (Bartels et al., 1997b), whereby

a CO molecule could be reliably transferred between the surface and

the tip. This ability adds chemical sensitivity to STM: with a tip having

a CO molecule at its apex, chemical contrast between otherwise similar

appearing adsorbates has been achieved: CO molecules on Cu(111)

always appear as depressions independent of the bias polarity when

imaged with a clean tip. However, when imaged with a tip having a

CO molecule at its apex, they appear independently of bias polarity as

protrusions. This inversion of shape allows chemical-sensitive imaging,

as shown in Figure 17–32. Here, O

and CO molecules were adsorbed

on the surface. Both adsorbates where imaged as depressions in the

STM images with a clean tip. Picking up the dark spot indicated

with a white arrow yielded a contrast reversal for most dark spots,

while some of them (black arrow) stayed the same. The conclusion

was that such spots originate from oxygen, while the others are CO

molecules.

Not only can adsorbates be manipulated, but single native substrate

atoms can be removed in a controlled manner from differently coordi-

nated sites of the substrate by using lateral manipulation techniques

(Meyer et al., 1997). This ability may be of importance in gathering

information about subsurface defects or to identify surface atoms with

a time-of-ﬂ ight analyzer (Weierstall and Spence, 1998) after transfer-

ring the atoms to the tip. Reversible lateral displacement of speciﬁ c Si

adatoms on the Si(111)-7 × 7 surface has been reported at low tempera-

tures (30–175 K) (Stipe et al., 1997a). A single adatom could be reversibly

displaced as an atomic switch and its position monitored with the tun-

neling current.

The continued miniaturization of electronic devices is leading to an

increasing interest in the application of single molecules in nanoelec-

tronics (Joachim et al., 2000). In this context, low-temperature STM is

a fundamental technique to study different molecular conformations

and to manipulate single molecules, bringing them in electronic contact

with atomically ordered nanoelectrodes. Extension of the lateral

Chapter 17 Low-Temperature Scanning Tunneling Microscopy 1109

manipulation mode from small to large molecules has been demon-

strated at low temperatures (Moresco et al., 2001). Although big mole-

cules have been positioned in a controlled way by room temperature

STM (Jung et al., 1996, 1997; Gimzewski and Joachim, 1999), the stabil-

ity and low noise level of the LT-STM are necessary to obtain detailed

and quantitative information about the manipulation process. Complex

organic molecules show a different translational movement under the

inﬂ uence of the STM tip than atoms or simple molecules. It has been

shown that Cu-TBPP, a specially designed porphyrin-based molecule,

cannot be moved at low temperatures by lateral manipulation in

constant-current mode. Therefore a constant-height mode for lateral

manipulation of these molecules was used, where the current signal

can be used to extract information about the internal movement of the

molecule under the action of the tip (Moresco et al., 2001). The central

group of a C

molecule (known as Lander) has been used as a

model system for a molecular wire. The Lander molecule has the same

molecular legs as Cu-TBTT, but it has a central polyaromatic molecular

wire instead of a central porphyrin ring (Figure 17–33). The molecule

was designed as a central molecular board lifted above the substrate

by four legs and should act as a short (1.7-nm-long) molecular wire

when contacted to an atomic step (Langlais et al., 1999). In an LT-STM

study the conformational changes induced in the Lander upon adsorp-

tion on Cu(001) have been investigated (Kuntze et al., 2002). It has been

found that the legs of adsorbed Lander molecules are rotated and

deformed. This leg rotation results in a lowering of the central wire to

Figure 17–32. Chemical contrast with functionalized tip at 15 K. (Top) STM

image of oxygen and CO on Cu(111) obtained with a clean metal tip. All adsor-

bates appear as indentations (dark spots) in the image. (Bottom) Same part of

the surface imaged after picking up the adsorbate marked with a white arrow.

All adsorbates imaged as protrusions correspond now to CO molecules,

whereas the adsorbate marked by the black arrow still appears as an indenta-

tion and is therefore identiﬁ ed as oxygen. The fact that the appearance of CO

changes when imaged with a CO molecule at the tip has been veriﬁ ed on a

surface where only CO molecules where adsorbed. (Form Bartels et al.,

1997b.)